There are basically two types of laboratory pipette:

• Air displacement
• Positive displacement

Air displacement pipettes are most commonly used in the laboratory for volumes greater than 100µl.

The Principles of Air displacement pipetting –

• Air displacement is adequate for any water based and non viscous fluids
• There is an air gap between the fluid and the plunger so there is no direct contact with the instrument
• The pipette can dispense either by forward or reverse pipetting techniques depending on the required accuracy and precision

Forward Pipetting technique –

Most commonly used pipetting technique that expels an exact volume of liquid from the pipette tip repeatedly. Careful technique is essential as the plunger is only depressed as far as the first stop when drawing up liquid and dispensing it. The technique must be reproducible and with smaller volumes this can be difficult to achieve.

 

 

Procedure:

• Depress plunger to first stop
• Immerse tip vertically into liquid and smoothly release the plunger
• Depress plunger button to first stop to dispense liquid into receiving vessel
• Depress button fully to second stop to expel the remaining liquid from the tip

 

 

 

Reverse Pipetting Techniques

Reverse pipetting techniques are only possible with air displacement pipettes. The liquid is aspirated in excess of the selected volume but only a set volume is dispensed. This improves precision with liquids such as viscous, volatile or foaming samples, and with smaller volumes below 100µl.

 

 

Procedure:

• Depress plunger button to second stop
• Immerse tip vertically into the liquid and smoothly release plunger button
• Depress plunger to first stop only to release liquid into vessel
• Discard remaining liquid by depressing to second stop

 

 

 

Recommended Pipetting Procedures –

 

•       Pre- wet tip x 2 before dispensing
•       Dip the tip 2-4mm into the liquid
•       Smooth aspiration both touching and not touching the side wall, (sometimes used with micrototre plate assays)

 

 

Positive Displacement Pipettes

• preferred for viscous, volatile or corrosive samples
• Direct contact between plunger and sample
• Works by replacing both plunger and capillary for each sample
• Liquid is forced out by the plunger through the capillary when dispensing
• best for small volume precious samples

 

Camlab have a comprehensive range of both air displacement and positive displacement pipettes online from both Socorex and Eppendorf.